1. Field of the Invention
This invention relates to a lead-free solder alloy, and particularly to a lead-free solder alloy having excellent solderability when used for soldering electronic components to printed wiring boards by flow soldering.
2. Description of the Related Art
Printed circuit boards are used in a wide range of electrical and electronic equipment including home electrical appliances such as televisions, videos, refrigerators, and air conditioners, as well as office or home electronic equipment such as personal computers, printers, and copying machines. Typically a printed circuit board includes a number of electronic components such as LSI's, IC's, transistors, registers, and capacitors secured to a printed wiring board by soldering.
The solder to be employed for this purpose is selected taking into consideration the various properties and cost of the solder. Solder wettability or solderability on the surfaces of electronic components and printed wiring boards is one of the most important properties of a solder. If soldering is performed with solder having poor wettability, the resulting soldered joints may include soldering defects such as non-wetting, bridges, and voids.
Sn—Pb solders have long been used for soldering electronic components to printed wiring boards due to their low soldering temperatures, good solderability or solder wettability, and low cost. In particular, a 63% Sn—Pb solder, which is called a Sn—Pb eutectic solder or simply a eutectic solder due to its alloy composition near the eutectic composition for Sn—Pb alloys (61.9% Sn—Pb), is used in a wide variety of soldering applications, since it has a narrow solidification temperature range (which is the difference between the liquidus and solidus temperatures of the alloy) and can form reliable soldered joints. (In this specification, unless otherwise specified, the percent of an element in an alloy composition refers to mass percent or “wt %”.)
When electrical or electronic equipment is discarded, it is usually disassembled to recover plastic parts such as housings and metallic parts such as chassis for recycling. However, printed circuit boards in the discarded equipment are not suitable for recycling, since they contain both metallic portions and plastic portions combined in a complicated manner. Therefore, in many cases, printed circuit boards removed from disassembled equipment are shredded and buried underground as industrial waste of a stabilized type.
In recent years, however, underground burial of lead-containing wastes including printed circuit boards has become an environmental problem. When the buried lead-containing wastes come into contact with acid rain (rain having a high acidity due to dissolving oxides of sulfur and nitrogen present in the atmosphere), the acid rain can dissolve lead from the wastes, and the dissolved lead can contaminate underground water. There is the concern that such contaminated water may cause lead poisoning if it is drunk by humans for long periods. To eliminate such environmental concerns, there is now a demand in the electronics industries for lead-free solders.
Lead-free solders which have been developed to date are based on Sn and contain one or more additional elements such as Cu, Ag, Bi, and Zn. Typical alloy compositions of lead-free solders are binary alloys such as Sn-0.7% Cu, Sn-3.5% Ag, Sn-58% Bi, and Sn-9% Zn, each having a composition which is the same as or close to the eutectic composition for the binary alloy system. Depending upon the use, additional alloying elements may be added to obtain a ternary or higher alloy.
Each of the above-mentioned lead-free solders has its own problems. For example, a Sn—Zn solder such as a Sn-9% Zn solder has the problem that Zn is highly susceptible to oxidation, resulting in the formation of a thick oxide film on the solder. As a result, wettability becomes poor if soldering is carried out in the air. In addition, when used in flow soldering, a Sn—Zn solder causes the formation of a large amount of dross, which causes difficult problems with respect to practical application of the solder.
With a Sn—Bi solder such as a Sn-58% Bi solder, the formation of dross during flow soldering is not a large problem, but due to the presence of a large proportion of Bi, which has poor ductility, the solder is brittle and has poor mechanical strength. Therefore, soldered joints formed from this solder may not be sufficiently reliable. There is a tendency for the mechanical strength of a Sn—Bi solder to decrease as the proportion of Bi increases.
At present, the lead-free solders which are considered most practical are Sn—Cu solders such as Sn-0.7% Cu, Sn—Ag solders such as Sn-3.5% Ag, and Sn—Ag—Cu solders (e.g., Sn-3.5% Sn-0.6% Cu) in which a small amount of Cu is added to a Sn—Ag solder.
Sn—Cu solders such as Sn-0.7% Cu are inexpensive and their unit cost is comparable to that of conventional Sn—Pb solders. However, they have poor solder wettability.
On the other hand, Sn—Ag solders such as Sn-3.5% Ag and Sn—Ag—Cu solders such as Sn-3.5% Ag-0.6% Cu have relatively good solder wettability, and their mechanical strength is comparable or even superior to that of Sn—Pb solders. Thus, these solders are advantageous in their properties as a solder, but their cost is much higher than that of conventional Sn—Pb solders due to the presence of Ag, which is an expensive metal. If the Ag content in these solders is decreased in order to reduce costs, the wettability and the strength of the solders worsen.
Thus, there is a need for an improved lead-free solder which has the cost advantages of a Sn—Cu solder but has improved properties, and particularly improved wettability.